1. Field of the Invention
This invention relates to a surface-treated magnetic powder with excellent oxidation resistance and excellent moisture resistance, and is also relates to a resin-bonded permanent magnet composition containing the same.
2. Description of the Prior Art
With rapid technological development in the field of electronics, electric household appliances and electronic equipment such as office automation equipment have been made miniature, light, and thin for the purpose of saving energy and resources. For smaller electric household appliances and electronic equipments, various parts used therein must be made smaller. For example, a motor used for a tape recorder of small size requires a small permanent magnet with excellent magnetic properties. In general, such a magnet must have a complicated form with precise dimensions.
However, conventional permanent magnets such as sintered magnets and cast magnets are not appropriate for use in smaller devices because their mechanical properties such as impact resistance and tensile strength are poor, as is their workability. Therefore, a resin-bonded permanent magnet (a so-called plastic magnet) obtained by the molding of a magnetic powder with a resin binder has been developed. Resin-bonded permanent magnets can be obtained easily by conventional molding methods. Also, a small permanent magnet with relatively large magnetic force can be obtained by selection of an appropriate magnetic powder. Molding materials (i.e., magnetic powders and binders), molding machines, technique to disperse magnetic powders into binders, and filling techniques of molding materials into molding machines have been improved recently, and the effective use and consumption of resin-bonded permanent magnets have therefore increased.
For the preparation of resin-bonded permanent magnets, ferrite-type or alnico-type magnetic materials have been usually used. However, in recent years, magnetic materials with an extremely high maximum energy product (for example, samarium-cobalt alloys) have been developed and are used widely to prepare small resin-bonded permanent magnets with relatively large magnetic force.
However, magnetic material of a samarium-cobalt alloy is very susceptible to oxidation, so it is in danger of burning during molding if not treated appropriately. Further, samarium, one of the components of this magnetic material, is usually found in only very small amounts among the rare earth metals in ore. Thus, the amount of samarium supplied depends upon the demand for other rare earth metals that are contained in the ore. It is expensive to separate and purify samarium. Cobalt also is expensive, and its supply is not steady because it is a strategic material.
In such circumstances, attempts to develop alternative magnetic materials that have a large magnetic force and that can be produced at a low price have been made. For example, Japanese Laid-Open Patent Publication No. 59-211549 discloses a magnetic material of the neodymium-iron-boron type that has extremely large magnetic force. This magnetic material can be obtained at low price because it contains a large amount of iron, which is widely available.
However, this type of magnetic material is also readily oxidized although it is less susceptible to oxidation than magnetic materials of the samarium-cobalt type. Further, because its major component is iron, the magnetic material will be corroded and rust in the presence of water. For example, when a resin-bonded permanent magnet prepared by the molding of this powdered magnetic material with a binder is used under the circumstances of relatively high humidity, rust will form on the surface of the magnet material and on the internal surfaces of the small spaces that are present in the magnet. As a result of the generation of rust, the magnetic force of the resin-bonded permanent magnet will decrease greatly with time. Moreover, rust inside the magnet will destroy the magnet itself. Thus, when a resin-bonded permanent magnet is used as a part of a motor, normal operation cannot be achieved because of rust.
To solve these problems, it has been proposed that the surface of such a powdered magnetic material be treated with phosphoric acid, chromic acid, etc., which are well known rust preventing agents for iron. However, because iron that is present near the surface of the magnetic powder will react with phosphoric acid or chromic acid, which changes the composition of the magnetic material of the powder a resin-bonded permanent magnet with large magnetic force cannot be prepared from the magnetic material. Further, because the magnetic powder retains the anti-rusting effect of treatment for only a short time, an additional resin layer must be formed on its surface in order to provide sufficient anti-rusting effect, which leads to several disadvantages such as poor magnetic properties, higher price, etc.
Japanese Laid-Open Patent Publication No. 62-152107 discloses a magnetic powder for a resin-bonded magnet, prepared by the formation of a layer of silicic acid anhydride or a silicate on the surfaces of a magnetic powder of the samarium-cobalt type to protect the powder from oxidation. The formation of the layer will improve its oxidation resistance. However, the layer of silicic acid anhydride has fine pinholes through which the magnetic powder is exposed to the air. When the magnetic powder with the coated layer is used under the circumstances of high humidity, some of the layer will dissolved into water because of its water solubility, so that the magnetic powder comes into contact with the water in the air directly. Therefore, the coated layer of silicic acid anhydride or silicate does not provide a magnetic powder with satisfactory anti-rusting properties.
To prevent rusting, an attempt has also been made to coat the surface of resin-bonded permanent magnets with a rust preventing agent or a resin. However, the resin-bonded permanent magnet should be coated with a layer about 20 .mu.m or more thick to provide satisfactory anti-rusting effects. The resulting resin-bonded permanent magnet with such a thick layer cannot, however, be used for precision parts with a complicated form.